Device and method for measuring relative concentration changes in gas stream components

ABSTRACT

A device for detecting and measuring fluctuations on a continuous basis and at most any concentration level, in a flowing gas stream, of a reactive component such as, oxygen, chlorine, bromine, iodine, sulfur dioxide, sulfur trioxide, hydrogen sulfide, carbon monoxide and compatible mixtures thereof utilizes as the detecting element a concentration cell made from two electrodes formed of substantially identical metal, a reference electrode and a sample electrode, at least the latter being foraminous in nature, an aqueous electrolyte containing a mobile soluble ion, the electrolyte contacting substantial areas of both electrodes, a substantially constant atmosphere or a lack of any atmosphere around the reference electrode, an enclosed zone around the sample electrode and means for bringing the gas stream to and from the enclosed zone. No impressed voltage or amperage is applied. The small potential difference across the electrodes generated by chemical equilibrium shifts in the concentration cell caused by fluctuations in the gas stream composition are measured by a sensitive Wheatstone bridge or a microammeter or other sensitive detection means.

United States Patent Hamilton DEVICE AND METHOD FOR MEASURING RELATIVECONCENTRATION CHANGES IN GAS STREAM COMPONENTS [75] Inventor: CharlesEugene Hamilton, Midland,

Mich.

[73] Assignee: The Dow Chemical Company,

Midland, Mich.

[22] Filed: Feb. 5, 1971 [21] Appl. No.: 112,980

[52] U.S. Cl 204/1 T, 204/195 R [51] Int. Cl. G01n 27/46 [58] Field ofSearch 204/ 195 R, 1 T

[56] References Cited UNITED STATES PATENTS 2,278,248 3/1942 Darrah204/195 R 2,414,411 1/1947 Marks 204/195 R 2,517,382 8/1950 Brinker eta1. 204/195 R 2,651,612 9/1953 Haller 204/195 R 2,805,191 9/1957 Hersch204/195 R 2,992,170 7/1961 Robinson 204/195 R 3,005,758 10/1961Spracklen et a1. 204/195 R 3,258,415 6/1966 Kordesch 204/195 R 3,315,2714/1967 Hersch et a1 204/195 R 3,329,599 7/1967 Brewer 204/195 RJanv,a/e2 ,sfream ll 11 i1 12 Primary Examiner-T. Tung Attorney, Agent,or Firm-Edward E. Schilling [57] ABSTRACT A device for detecting andmeasuring fluctuations on a continuous basis and at most anyconcentration level, in a flowing gas stream, of a reactive componentsuch as, oxygen, chlorine, bromine, iodine, su1fur dioxide, sulfurtrioxide, hydrogen sulfide, carbon monoxide and compatible mixturesthereof utilizes as the detecting element a concentration cell made fromtwo electrodes formed of substantially identical metal, a referenceelectrode and a sample electrode, at least the latter being foraminousin nature, an aqueous electrolyte containing a mobile soluble ion, theelectrolyte contacting substantial areas of both electrodes, asubstantially constant atmosphere or a lack of any atmosphere around thereference electrode, an enclosed zone around the sample electrode andmeans for bringing the gas stream to and from the enclosed zone. Noimpressed voltage or amperage is applied. The small potential differenceacross the electrodes generated by chemical equilibrium shifts in theconcentration cell caused by fluctuations in the gas stream compositionare measured by a sensitive Wheatstone bridge or a microammeter or othersensitive detection means.

20 Claims, 12 Drawing Figures mimmrza 1 s 1924 SHEEIiUFS HTTORA/EYPATENTEB FEB I 91974 Way. 11

INVENTOR.

ATTORNEY PAIENTED 3,793 1 58 sum 3 0F 3 damp 6 J fream v 60 INVENTOR. 1Char/es Eu; ene Harm/{0n W 2 Mn ATTORNEY DEVICE AND METHOD FOR MEASURINGRELATIVE CONCENTRATION CHANGES IN GAS STREAM COMPONENTS The method ofmeasuring fluctuations in composition of a gas stream comprises passingthe fluctuating gas stream over the juncture of a sample electrode andan aqueous electrolyte containing a mobile soluble ion, the sampleelectrode being in electrolytic contact, through the aqueouselectrolyte, with a reference electrode formed of a substantiallyidentical metal and the two electrodes being in electrical contactthrough sensitive electrical current or voltage detection means, andmeasuring the concentration cell effects as composition fluctuationsoccur.

BACKGROUND OF THE INVENTION 1. Field of the Invention The inventionrelates to galvanic apparatus and method of detecting, on a continuousbasis, qualitatively and quantitatively, fluctuating changes in thecomposition of a gas stream containing an electrolytesoluble, detectablenon-inert gas in a range from substantially 100 per cent base lineconcentration down to barely detectable concentrations of the gascomponent being measured. The invention particularly relates toapparatus and method of detecting and measuring gas stream compositionchanges with a galvanic cell containing substantially identicalelectrodes but without means for impressing a potential across theelectrodes, the sample electrode being semi-immersed in a commonelectrolyte, and the reference electrode being at least semi-immersed.

2. Description of the Prior Art A rather thorough discussion of galvanicanalysis methods and cells is set forth in Advances In AnalyticalChemistry and Instrumentation," Volume 3, John Wiley & Sons, lnc., NewYork, 1964, in the section at pp. 183-249 on Galvanic Analysiscontributed by P. Hersch, the historical descriptions being found at pp.185-188. Galvanic cells employing electrodes formed of dissimilar metalshave been employed to measure a number of oxidants in air but have beenfound to be insensitive to the most common oxidant oxygen. Hersch, inBritish Pat. No. 913,412 published Dec. 19, 1962, describes a cellemploying like electrodes, which are semi-immersed in a commonelectrolyte, to provide a three phase boundary between gas, electrolyteand sample electrode and utilizes an applied potential sufficient toreduce oxygen, but insufficient to decompose water. The use of anapplied potential is essential to Herschs method. The Hersch cell ismore sensitive than most of its predecessors but it suffers from thedisadvantages that it is not suited to detecting small changes at highoxidant level, nor in exhibiting the'sensitivity now desired in an evenmore sophisticated world of pollution control technology.

OBJECTS OF THE INVENTION It is a principal object of the invention toprovide galvanic apparatus for and a method capable of continw ousdetection and quantitative measurement of small fluctuations incomposition in the parts per million' stream in which the othercomponent or the mixture of remaining components is relativelynon-reactive with the electrodes, the electrolyte, and, the membrane, ifany, used to retain electrolyte in contact with the electrodes.

Another object of the invention is to provide method and apparatuscapable of continuously detecting and measuring fluctuations as small asparts per million in concentration of a reactive gas in a gas stream inwhich the reactive gas is present in a concentration above about 50 percent and as high as 98 per cent by volume.

A further object of the invention is to provide a galvanic cell for thedetection and measurement of small fluctuations in composition of a gasstream which employs relatively low cost electrodes in a compactcomposite cell.

STATEMENT OF THE INVENTION The invention is based on the discovery thatupon providing a calibrated galvanic gas analysis cell utilizing areference electrode and a sample electrode each formed of a like metal,at least the latter being foraminous, the sample electrode beingsemi-immersed and the reference electrode at least semi-immersed. in acommon aqueous electrolyte containing a mobile soluble ion, maintaininga substantially constant environment around the reference electrode andpassing a gas stream containing a fluctuating level of anelectrodereactive gas over the sample electrode whereby the reactive gascontacts the juncture of electrolyte and sample electrode forming athree-phase boundary, fluctuations in concentration of said reactive gascreate a concentration cell resulting in a fluctuating potentialdifference between the sample and reference electrodes that isproportional to the magnitude of the fluctuations in the concentrationof the reactive gas component of the gas stream and upon measuring thepotential difference fluctuations of the calibrated cell the gasconcentration fluctuations are quantitatively detected and measured.

THE DRAWINGS The present invention will be better understood uponbecoming familiar with the following description, reference being had tothe drawings in which:

FIG. 1 is a view of the present device in side elevation partly brokenaway and in section and partly schematic as to the meter circuit; V

FIG. 2 is an enlarged fragmentary view in section of a portion of thecell shown in FIG. 1 showing details of construction;

FIG. 3 is a fragmentary view of a device similar to that of FIG. 1 buthaving a different configuration for the reservoir and cell effluentconduit;

FIG. 4 shows another embodiment of the present device in side elevationpartly broken away and in section;

FIG. 5 is a transverse sectional view of the device in FIG. 4 takenalong line 5-5; I

FIG. 6 is an enlarged fragmentary view of part of a section of a portionof the cell shown in FIG. 4 showing details of the relationship of outerwall, hydrophobic liner, annular space, inner and outer wire helixes,the intermediate porous membrane, and the solid inner core;

FIG. 7 shows yet another embodiment of the present device in sideelevation partly broken away and in section;

FIG. 8 shows a foreshortened top view of still another embodiment of thedevice of the present invention, with the glass cover partly broken awayand with the electrolyte reservoir omitted;

FIG. 9 is a view in vertical section and partly schematic of the deviceof FIG. 8 taken along the line 99, but with the electrolyte reservoiradded;

FIG. 10 is an enlarged fragmentary view in section of a portion of theelectrode grids of FIG. 9 showing the relationship to the electrolytelevel in the device;

FIG. 11 is a fragmentary view partly broken away successively and insection of a portion of a device similar to that of FIG. 1, but havinggrid form electrodes instead of wire helix electrodes; and

FIG. 12 is a fragmentary view similar' to FIG. 11 showing a portion of adevice according to the invention with perforated sheet form electrodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred form of theapparatus of the invention appears in FIG. 1 in which is shown asubstantially vertically disposed galvanic cell indicated generally bythe reference numeral 10. The cell 10 is made up of two elongatedconcentric wire helixes l1, 12 which serve as electrodes, the innerhelix 11 being separated from the outer helix 12 by a wick-like membrane13, typically a bibulous paper, that, as a matter of convenience,extends above and below the helixes to make connections with a gassample inlet conduit 14 and outlet conduit 15, and the combination ofhelixes and paper sleeve is enclosed, from inlet conduit 14 to outletconduit 15, by a gas impervious shell 16.

The outlet conduit 15 extends into and through the bottom of a reservoir17 of aqueous electrolyte solution 18. If desired, the outlet conduit 15may take the form of the U-tube inlet shown in FIG. 3. The samplestream, as indicated in FIG. 3, may also be flowed upwardly through thecell if more convenient for the geometry of the sampling system, butbetter response and sensitivity are generally obtained with downdraftoperations.

Referring again to FIG. 1, it is essential that the bibulous papersleeve 13 extend down into the electrolyte 18 for at least about aquarter inch and preferably onehalf to three-quarters of an inch so thatthe sleeve can well serve its purpose as a wick carrying electrolyte upsubstantially all the way between the helically generated electrodes. Amorepositive seal of sleeve 13 to outlet conduit 15 is readily assuredby placing a rubber retaining ring or band 19 around the sleeve where itoverlaps and surrounds the conduit.

The cell 10 throughout most of its length is enclosed by the gasimpervious shell 16 which is conveniently a simple spiral wrapping of asubstantially gasimpervious pressure sensitive tape such as the modemsynthetic plastic electrician 's tape. Electrical leads 20, 21 extendthrough the wrapping 16 from theupper end of each helical electrode ll,12, respectively, and are electri cally connected through means 22 fordetecting and measuring small differences in electrical potentialbetween the Said electrodes, such means 22 and the electricalconnections thereto being represented schematically.

It is essential that the wick-like membrane 13 transports electrolyte 18up from the reservoir 17 throughout the major part of the length of thehelical electrodes 11 and 12 in order to provide a substantial length ofthree-phase boundary of gas, electrode, and electrolyte and thus achievesensitivity.

A portion of the wetted electrode combination is shown in thefragmentary sectional view of FIG. 2. Of particular importance is thethree-phase boundary of helical electrode, electrolyte and gas streamindicated at several points by the numeral 23. Liquid electrolytesolution wets each helix throughout at least a substantial portionthereof and the meniscus formed with adjacent turns of each helixdetermines the location of the three-phase boundary. The three-phaseboundary is actually a substantially continuous zone or line in the caseof each helical form electrode semi-immersed in electrolyte. The zoneconstitutes a reaction site for the electrode reaction. Such zones arediscontinuous or segmented in the case of grid or screen or perforatedsheet type electrodes. However, in each case there must be electricalcontinuity throughout each electrode from the electrolyte in thereservoir to the electrical lead connected to the potential differencemeasuring means.

The three-phase boundary, then,'is where the all important electrodereactions take place at the sample electrode and the extent and natureof such boundary greatly determine the sensitivity of the presentapparatus and method, while the length of the electrolyte path betweenthe two electrodes greatly affects response time.

The present apparatus and method are capable of detecting andquantifying extremely small rapid fluctuations in gas concentration,providing sensitive methods of measuring changes in electrical potentialare used. Thus, e.g., in a gas stream consisting of a mixture ofhydrocarbon gases and containing oxygen as an impurity, concentrationchanges of the order of 0.5 nanograms of oxygen impurity per liter permillisecond are measurable with an oscilloscope read out on the metermeasuring changes in potential difference. Moretypically, changes of 500nanograms per liter per second or greater change per greater time periodare measurable.

past the sample electrode and causing changes at the three-phaseboundary. 1

Typical of the half-cell or electrode reactions are the cathodicreactions of oxygen in neutral or alkaline aqueous medium:

0 2H O 4e 40H and acidic aqueous medium: 0 4 l-I 4e 2H O Other half cellreactions are:

C1 +2e 2C1 C0 e CO (carbonyl complex) in each case, for the gascomponent to be detectable, a cathodic reaction must take place rapidlyenough to create sufficient potential difference to permit detection ofwhatever order of magnitude of change is essential for utility.

The electrolyte solution as the term implies, should contain anelectrolyte supplying ions mobile enough to transmit current quickly andelectrolytically. The solution must be capable of taking up the gas tobe detected and facilitating a cathodic reaction. The electrolyte solution, the wick or membrane and the metal of the electrodes are eachcoordinately selected with care to avoid any interfering reactions whichwould hide or distort the potential difference arising from thegaselectrode reaction. In addition, these items are also chosenappropriately according to the gas stream and component to be detectedso that there is a detectable reaction between the gas and the metal ofthe electrode but no rapid destructive reaction with the electrodes. Anexample of a destructive reaction to be avoided is the reaction ofaluminum metal in a sodium hydroxide solution. Preferably the gas streamdoes not react at all with any of the other parts of the cell except theelectrolyte solution and then only to the extent that it participates inthe electrode reaction.

Examples of suitable combinations of electrode metal and electrolyte aretabulated as follows:

Suitable Combinations of Electrode Metal and Electrolyte MetalElectrolyte Silver NaOH Silver NaCl Silver AgNO; Platinum NaOH PlatinumNaCl Platinum PtCl, Palladium PdC l ,'2H O Copper CuCl, CopperCuCl,-2NH.C1 Copper CuSO. Aluminum (Al),(SO Aluminum AlCl;

lron NaCl lron FeCl Stainless Steel (316) NaOH Stainless Steel (3 l6)NaCl lectivity is accomplished by an appropriate choice of electrodemetal where differential reactivity is possible. In general, one willnot be interested in the total change in a mixture of components in agas stream, but will hold the concentration of all components but onesteady and measure the change in the single component with the presentapparatus and process. The following oxidizing gases are readilymonitored in the listed gas streams:

'HX hydrogen halide The following reducing gases are readily monitoredin the listed gas streams:

Reducing Gas Gas Stream Hydrogen Sulfide Methane Hydrogen SulfideMethane-Carbon Dioxide Mixture Carbon Monoxide Carbon Dioxide CarbonMonoxide Air Carbon Dioxide The Inert Gases, e.g. He

The gas stream to be monitored is brought into intimate contact with thesample electrode so that the three-phase boundary condition isfulfilled. However, the gas stream must be kept out of contact with thereference electrode or there can be little concentration cell effect.

The reference electrode must be surrounded by a relatively constant orunchanging environment in order to detect the changes at the sampleelectrode and measure the amount of change. The environment around thereference electrode may be electrolyte and (l) a flowing gas ofconstant, known composition, (2) electrolyte and a static gasenvironment, (3) eletrolyte, and, void spaces filled with vapor from theaqueous electrolyte including, in some cases, gas from the sample streamdiffused through the electrolyte, or (4) electrolyte only, depending onthe type of cell construction selected.

It must constantly be kept in mind that measurements by the presentmethod are relative and that the thing which is detected and measured,is change in concentration of a gas component. On steadily passing agiven gas stream of highly uniform composition through the apparatus ofthe invention, a steady base line is drawn by a recorder attached to ora part of the potential difference measuring means. Electrode reactivecomponents in the gas stream will enter the electrolyte and diffuse tothe reference electrode so that a steady state is reached at which thereis no concentration cell effect, and consequently no potentialdifference between the electrodes. At this point, increasing theconcentration of the reactive gas component makes one electrode thecathode and the other the anode. But, decreasing the concentration ofthe same component from the said steady state level reverses theelectrode relationships. As a consequence, fluctuations of gasconcentration'both above and below the steady state or base line valueresults in recorder peaks both above and below the base line. This maybe disconcerting if not understood, and moreover, cuts the O toadjustment on the recorder at least in half on at least one side of thebase line.

The problem of fluctuations across the base line is avoided by providingas the gaseous part of the environment around the reference electrode, astatic or a dynamic gas atmosphere sufficiently different from thesample stream in concentration of reactive component as to assurefreedom from reversal of electrode designations. Thus, by propermanipulation of the reference electrode environment, the problem offluctuations across the base line is avoided.

As herein illustrated by the various embodiments, the sample electrodestructure should be gas permeable substantially throughout the extent ofthe electrode structure while the reference electrode may be gaspermeable or impermeable. Thus, the electrodes may each be a wire helixor a fine wire screen or sieve, i.e., with small openings as illustratedin the fragmentary sectional view of FIG. 1 1, showing a cellarrangement similar to FIG. 1, or, a highly perforated or foraminousmetal sheet as illustrated in the fragmentary sectional view of FIG. 12,while the reference electrode may generally be the same or take the formof an unperforated sheet, the latter being applicable where thereference atmosphere is really zero atmosphere and no threephaseboundary is required at the reference electrode.

The gas permeable electrodes should have a total length of three-phaseboundary in the range of about 340 to 350 inches per square inch ofelectrode metal surface exposed to the gas stream.

The electrodes may be disposed closely adjacent concentrically, or, faceto face, if planar, or, disposed in separate gas compartments butcontacted throughout a substantial surface area by the same electrolyte.Contacting includes complete immersion of the reference electrode andsemi-immersion of both electrodes with or without the aid of a bibulouspaper or other wicklike membrane in contact with the electrode.

The most satisfactory all around performance is gotten from a cell ofthe invention utilizing a gas permeable sample electrode made up into ahelix or screen from wire about 1 to 5 mils in diameter and with abibulous membrane about 0.5 to 1 mil thick separating the electrodes.Providing l for a long three-phase boundary, and (2) for closed-spacedelectrodes by utilizing a thin bibulous membrane, facilitates obtaininga rapid, substantial, response from the cell, the path length defined byelectrode spacing being particularly critical. On the other hand, usingthe larger spacing, e.g., on utilizing a thicker membrane between theelectrodes, per mits the more accurate measurement of slow changes ingas stream composition since the diffusion of the gas component takesplace more slowly across the' longer electrolyte path through the largerspacing from the sample electrode to the reference electrode;

As another means of keeping the electrolytic path short the viscosity ofthe electrolyte and the wettability of the electrodes are controlled ormodified so that the electrolyte miniscus on the electrodes is not farfrom the membrane, e.g., not more than about one-third of the wirediameter, the latter being generally in the range of about I to mils.

In each case the sample and reference electrodes are formed of the samemetal or alloy, or substantially so, in order that the electricalpotential therebetween is substantially zero when the composition of the.gas

stream under study is steady andunchang'ing so that an appropriate baseline reading may be obtained for purposes of accurately measuring rapidsmall fluctuations in gas concentration measurements.

Another embodiment of the cell of the invention indicated generally bythe numeral 24 is shown in Figures 4, 5 and 6 in which are illustrated aconcentric arrangement of the helical electrodes 25, 26 with the innerbeing the reference electrode 25-closed-spaced around a supporting solidrod 27 and surrounded by a bibulous paper sleeve 28 which is, in turn,surrounded by the sample electrode 26. The electrode assembly sits in asubstantially rigid cylindrical shell 29 in which an annular space 30separates the sample electrode 26 from the plastic liner 31 for theshell 29.

The lower end of the cylindrical shell 29 serves as a reservoir for theelectrolyte 32 which may be emptied or replenished through a valvecontrolled side arm 33. The plastic liner 31 may be omitted if desired,but tends to keep electrolyte 32 from wetting the wall of the shell 29and creeping up the wall where it is subject to the influence of thesample gas stream which tends to drive the electrolyte up the wall andto cause coalesced droplets to bridge the annular space. A sample streamto be analyzed or monitored is introduced through an inlet line 34connected to the cylindrical shell 29 just above the level ofthereservoir of electrolyte 32. The sample gas stream moves up throughthe-annular space 30 and exits through the outlet line 35 which isconnected to the cylindrical shell 29 near the upper end thereof. Thesample gas stream may also be flowed downwardly through the cell, ifdesired.

The cell may be closed at the top in any suitable manner so long asprovision is made to support and space the solid rod 27 and helicalelectrodes 25 and 26 and to bring out the electrode leads 36, 37 fromthe enclosure, here, as shown, by bringing the leads out through astopper 38 that has been centrally pierced to slide over the solid rod27.

In an additional embodiment of the apparatus of the invention shown inFIG. 7, the sample electrode 39 and the reference electrode 40 are eachprovided in cylindrical shell form, here as wire helixes, though theelectrodes may also be in the form of wire grids or screens,

or perforated thin metal plates. Each electrode is dis posed in arespective area 41,42 of the U-shaped tube 43. Each electrode 39, 40 isdisposed concentrically and snugly about a wick-like sleeve 44, 45,generally of bibulous paper, which has been slipped onto a supportiverod or core 46, 47. Theelectrodes 39, 40 are positioned vertically justabove the electrolyte 48 in the lower part of the U-tube 43, while thewick-like sleeves 44, 45 extend down the electrolyte 48. The cores 46and 47 and accompanying electrodes and sleeves are respectivelysupported and spaced centrally in the arms 41, 42 of the U-tube 43 byinsertion into stoppers 49, 50, substantially closing the said arms ofthe U-tube. It is necessary to vent the gas streams entering each arm ofthe U-tube through respective side arms 51, 52, and this may be done byproviding a borehole 53, 54 in each stopper or, ifdesired, by providinga sidearm to each arm of the U-tube adjacent the stoppers.

It is not necessary to provide a flowing gas atmosphere around thereference electrode 40 and generally it will be preferred to close offor eliminate the side arm 52 and the vent 54, thus creating a'static,fixed atmosphere .in the arm 42.

The electrical leads 55, 56, one from each electrode, are convenientlybrought out through the stoppers 49, 50, adjacent the cores 46, 47, asshown, or at the perimeter of the stoppers, and are connected to aninstrument (not shown) for measuring small differences in electricalpotential.

The U-tube type cell exhibits a sluggish response, requiring up to 15minutes or more to react to change, presumably because of the relativelylong electrolytic path. Such a cell is useful in detecting slowerchanges which tend to be not detectable with the fast response cells.This is especially to be expected with the fast cells having only adiffused atmosphere around the reference electrode since the nature ofthe electrolyte composition and membrane permeability effect the gasdiffusion rate to equal the drift with the slow change and thus bothelectrodes would appear to be seeing like concentrations throughout theperiod of slow change.

Yet another embodiment of the apparatus of the invention is shown inFIG. 8, 9 and 10. In this embodiment the electrodes 57, 58 are planargrids and there is no need to employ a membrane since the superposed,slightly spaced apart, electrodes are supported in a horizontal positionby buoyant elements 59 with the lower or reference electrode 58 fullysubmerged in the electrolyte 60 and the sample electrode 57 verycritically and carefully partly submerged so that a substantial extentof three-phase boundary length is provided. The buoyant elements 59 maybe pieces of cork or other low density natural material but ispreferably a cellular synthetic polymeric material, such as foamedpolyurethane, with substantially all closed cells, and is desirablyresistant to deterioration upon long immersion in the electrolyte 60.The cell proper, indicated generally by the numeral 61, is a shallowrectangular vessel 62, preferably with a cover conveniently opened, suchas the hinged cover 63 shown. An inlet connection 64 at one end of thevessel admits sample stream as well as make-up electrolyte in the eventof evaporative losses, although the electrolyte may be supplied througha separate inlet if desired. Make-up additions are an essential part ofthe critical function of keeping the electrolyte level substantiallyconstant in the cell.

The sample stream passes over the electrolyte grid 57, filling the space65 above said grid, and moving on out through the exhaust outlet 66. Theelectrical leads 67, 68 from the respective electrodes 57, 58 areconveniently led out of the cell through an opening 69 in the exhaustoutlet 66 and normallyare connectedto a meter (not shown).

The present cell can detect small changes in concen-' tration in acomponent present in high concentration more readily than a cell usingdissimilar electrodes and a polarographic current since there is nobucking potential employed to balance the polarographic current at themeter. Where polarographic current is employed, the noise level in thebucking potential at the meter becomes equivalent in magnitude tochanges in current due to small changes in gas concentration and thelatter cannot be readily detected amongst the noise peaks. v

To avoid undue drying out of the wick or membrane in working with arelatively low humidity gas stream, it is preferred to scrub the gasstream with an inert or gas-saturated aqueous scrubberahead of thedetection cell of the invention and to supply the gas stream to the cellat substantially constant humidity.

What is claimed is: r 1. The galvanic cell device for detecting andmeasur- 5 ing fluctuations in the concentration level, in a flowing gasstream, of an electrode reactive gaseous component which comprises:

measuring means and sensing means for an autogenously generatedelectrical potential difference; the sensing means consistingessentially of a substan tially enclosed cell having an inlet and anoutlet, said cell having disposed therein a sample electrode and areference electrode each formed of substantially the same metal wherebythe electrical potenwhen the composition of said gas stream is steadyand unchanging, said cell having a liquid reservoir portion adapted tohold an aqueous electrolyte solution at least closely adjacent to eachelectrode whereby the electrodes are readily connected electrolytically,said cell having means for directing the flowing gas stream over thesample electrode during passage from the inlet to the outlet and meansfor keeping the gas stream out of contact with l the reference electrodeand (2) the electrolyte in the reservoir;

and the sample and reference electrodes being electrically connected viasaid measuring means for the autogenously generated electrical potentialdifference, and said sample and reference electrodes when connectedelectrolytically and further connected electrically in series to saidmeasuring means constituting a concentration cell;

said cell being without means for impressing a potential across theelectrodes;

and the sample and reference electrodes being adapted to autogenouslygenerate an electrical potential difference only upon fluctuations inthe gas stream composition.

2. The galvanic cell device for detecting and measuring fluctuations inthe concentration level, in a flowing gas stream, of a gaseous componentselected from the groupconsisting of oxygen, chlorine, bromine, iodine,sulfur dioxide, sulfur trioxide, hydrogen sulfide and carbon monoxideand a mixture thereof which comprises:

measuring means and sensing means for an autogeneously generatedpotential difference; I the sensing means consisting essentially of asubstantially enclosed cell adapted to permit passage of the gas streamtherethrough and containing an aqueous electrolyte solution in additionto first and second electrodes each formed of the same metal theelectrodes being connected (1) electrically, via the potentialdifference measuring means and (2) electrolytically,- via the aqueouselectrolyte solution and the cell being without means for impressing apotential across the electrodes;

said first electrode being a sample electrode having a foraminousstructure and said electrode having a surface adapted to be exposed tosaid flowing gas stream and to have zones in which gas stream,electrolyte solution and metal surface meet to form a three-phaseboundary;

'said second electrode being a reference electrode adapted to be incontact with anunchanging envitial between the electrodes issubstantially zero ronment with respect to the gas stream component tobe detected and measured; said aqueous electrolyte solution containing amobile soluble ion and being not destructively reactive with theelectrode metal under test conditions;

and means for bringing the flowing gas stream to and from the sampleelectrode.

3. The device as in claim 2 in which each electrode is planar in form,the electrodes being substantially horizontally disposed, slightlyspaced apart, substantially parallel and co-aligned and suspended in theelectrolyte solution, one of the electrodes being foraminous and onlypartly submerged and the other electrode being entirely submerged.

4. The device as in claim 2 in which the electrodes are substantiallyvertically disposed and separated by a porous membrane with which eachelectrode is in substantial contact, the electrodes being adjacent andabove the electrolyte solution, the membrane extending into and beingwetted by and having wicking action toward the said electrolytesolution, and the electrolyte solution being taken up substantiallythroughout the extent of the membrane.

5. The device as in claim 4 in which the electrodes and membrane areeach substantially in the form of contiguous concentric cylindricalshells.

6. The device as in claim 5 in which the sample stream is adapted toflow through a space defined by the inside surface of the innermostconcentric shellform electrode which is the sample electrode.

7. The device as in claim 5 in which the innermost concentric shell-formelectrode is the reference electrode and fits around a solid coresupport, the means for bringing the gas stream to and from the sampleelectrode includes a body shell surrounding the concentric array ofelectrodes and membrane and spaced apart therefrom to provide an annulusthrough which the gas stream is adapted to flow, the outer electrode inthe array being the sample electrode.

8. The device as in claim 4 in which the electrolyte solution is about a5 to per cent by weight aqueous solution of'an inorganic electrolyte andthe membrane is about 0.5 to about 1 mil thick.

9. The device as in claim 2 in which the first electrode has zonesthereof in which the gas stream, the electrolyte solution and the metalsurface of the electrode itself meet along a line that constitutes athreephase boundary and the ratio of the total length of three-phaseboundary to the gas contact area of the electrode is at least about 340,length and area being expressed in terms of the same basic units oflinear measurement. i

10. The device as in claim 2 in which each electrode is disposed in arespective arm of a U-tube having electrolyte solution therein, theelectrodes each being adjacent and above the electrolyte solution andeach electrode having a wick-like porous membrane in substantiallycoextensive contact therewith, each membrane extending into. theelectrolyte solution and being wetted thereby, and the electrolytesolution being taken up substantially throughout the extent of eachmembrane.

- II. The device as in claim 2 in which the potential differencemeasuring means is a potentiometric apparatus.

12. The device as in claim 2 in which the potential difference measuringmeans is an amperometric apparatus.

13. The device as in claim 2 in which the metal of which the electrodesare made is a metal selected from the group consisting of silver,platinum, palladium, copper, aluminum, iron, 316 stainless steel, andlead.

14. The device as in claim 2 in which the electrodes are each made ofsilver and the electrolyte solution is an aqueous solution of a compoundselected from the group consisting of NaOH, NaCl and AgNO 15. The deviceas in claim 2 in which the electrodes are each made of platinum and theelectrolyte solution is an aqueous solution of a compound selected fromthe group consisting of NaOH, NaCl and PtCl 16. The device as in claim 2in which the electrodes are each made of copper and the electrolytesolution is an aqueous solution of a compound selected from the groupconsisting of CuCl CuCl -2 NH Cl and CuSO 17. The device as in claim 2in which the, electrodes are each made of aluminum and the electrolytesolution is an aqueous solution of a compound selected from the groupconsisting of Al SO.,);, and AlCl 18. The device as in claim 2 in whichthe electrodes are each made of lead and the electrolyte solution is anaqueous solution of a compound selected from the group consisting ofPb(NO and Pb(C H O 19. The method of detecting the fluctuations in theconcentration level, in a flowing gas stream, of a gaseous componentselected from the group consisting of oxygen, chlorine, bromine, iodine,sulfur dioxide, sulfur trioxide, hydrogen sulfide and carbon dioxide anda mixture thereof which comprises:

providing a sample electrode and a reference electrode of substantiallythe same ,metal', a porous membrane, and a reservoir containing anaqueous electrolyte solution, the electrodes being separated by and eachin contact with the porous membrane, the membrane extending intoandbeing wetted by and haVing wicking action toward the electrolytesolution and the electrolyte solution being taken up substantially.throughout the extent of the membrane, said electrolyte solutioncontaining a mobile soluble ion and beingnot destructively reactive withthe electrodes, both electrodes being electri-.

cally connected via potential difference measuring means, conductingsaid gas stream over the sample electrode without contacting thereference electrode or the electrolytesolution in the reservoir, andmeasuring changes in potential difference be tween the electrodes as aconsequence of said fluctuations without impressing a' potentialacross-the electrodes. I

20. The method as in claim 19 in which the reference electrode isprovided with a controlled environment that contains a concentration ofthe said gaseous component such that the reference electrode does notalternatelyfunction as cathode and anode as a result of fluctuations inconcentration of the said gaseous component being detected in theflowing gas stream.

. a an

2. The galvanic cell device for detecting and measuring fluctuations inthe concentration level, in a flowing gas stream, of a gaseous componentselected from the group consisting of oxygen, chlorine, bromine, iodine,sulfur dioxide, sulfur trioxide, hydrogen sulfide and carbon monoxideand a mixture thereof which comprises: measuring means and sensing meansfor an autogeneously generated potential difference; the sensing meansconsisting essentially of a substantially enclosed cell adapted topermit passage of the gas stream therethrough and containing an aqueouselectrolyte solution in addition to first and second electrodes eachformed of the same metal the electrodes being connected (1)electrically, via the potential difference measuring means and (2)electrolytically, via the aqueous electrolyte solution and the cellbeing without means for impressing a potential across the electrodes;said first electrode being a sample electrode having a foraminousstructure and said electrode having a surface adapted to be exposed tosaid flowing gas stream and to have zones in which gas stream,electrolyte solution and metal surface meet to form a three-phaseboundary; said second electrode being a reference electrode adapted tobe in contact with an unchanging environment with respect to the gasstream component to be detected and measured; said aqueous electrolytesolution containing a mobile soluble ion and being not destructivelyreactive with the electrode metal under test conditions; and means forbringing the flowing gas stream to and from the sample electrode.
 3. Thedevice as in claim 2 in which each electrode is planar in form, theelectrodes being substantially horizontally disposed, slightly spacedapart, substantially parallel and co-aligned and suspended in theelectrolyte solution, one of the electrodes being foraminous and onlypartly submerged and the other electrode being entirely submerged. 4.The device as in claim 2 in which the electrodes are substantiallyvertically disposed and separated by a porous membrane with which eachelectrode is in substantial contact, the electrodes being adjacent andabove the electrolyte solution, the membrane extending into and beingwetted by and having wicking action toward the said electrolytesolution, and the electrolyte solution being taken up substantiallythroughout the extent of the membrane.
 5. The device as in claim 4 inwhich the electrodes and membrane are each substantially in the form ofcontiguous concentric cylindrical shells.
 6. The device as in claim 5 inwhich the sample stream is adapted to flow through a space defined bythe inside surface of the innermost concentric shell-Form electrodewhich is the sample electrode.
 7. The device as in claim 5 in which theinnermost concentric shell-form electrode is the reference electrode andfits around a solid core support, the means for bringing the gas streamto and from the sample electrode includes a body shell surrounding theconcentric array of electrodes and membrane and spaced apart therefromto provide an annulus through which the gas stream is adapted to flow,the outer electrode in the array being the sample electrode.
 8. Thedevice as in claim 4 in which the electrolyte solution is about a 5 to15 per cent by weight aqueous solution of an inorganic electrolyte andthe membrane is about 0.5 to about 1 mil thick.
 9. The device as inclaim 2 in which the first electrode has zones thereof in which the gasstream, the electrolyte solution and the metal surface of the electrodeitself meet along a line that constitutes a three-phase boundary and theratio of the total length of three-phase boundary to the gas contactarea of the electrode is at least about 340, length and area beingexpressed in terms of the same basic units of linear measurement. 10.The device as in claim 2 in which each electrode is disposed in arespective arm of a U-tube having electrolyte solution therein, theelectrodes each being adjacent and above the electrolyte solution andeach electrode having a wick-like porous membrane in substantiallycoextensive contact therewith, each membrane extending into theelectrolyte solution and being wetted thereby, and the electrolytesolution being taken up substantially throughout the extent of eachmembrane.
 11. The device as in claim 2 in which the potential differencemeasuring means is a potentiometric apparatus.
 12. The device as inclaim 2 in which the potential difference measuring means is anamperometric apparatus.
 13. The device as in claim 2 in which the metalof which the electrodes are made is a metal selected from the groupconsisting of silver, platinum, palladium, copper, aluminum, iron, 316stainless steel, and lead.
 14. The device as in claim 2 in which theelectrodes are each made of silver and the electrolyte solution is anaqueous solution of a compound selected from the group consisting ofNaOH, NaCl and AgNO3.
 15. The device as in claim 2 in which theelectrodes are each made of platinum and the electrolyte solution is anaqueous solution of a compound selected from the group consisting ofNaOH, NaCl and PtCl4.
 16. The device as in claim 2 in which theelectrodes are each made of copper and the electrolyte solution is anaqueous solution of a compound selected from the group consisting ofCuCl2, CuCl2.2 NH4Cl and CuSO4.
 17. The device as in claim 2 in whichthe electrodes are each made of aluminum and the electrolyte solution isan aqueous solution of a compound selected from the group consisting ofAl2(SO4)3 and AlCl3.
 18. The device as in claim 2 in which theelectrodes are each made of lead and the electrolyte solution is anaqueous solution of a compound selected from the group consisting ofPb(NO3)2 and Pb(C2H3O2)2.
 19. The method of detecting the fluctuationsin the concentration level, in a flowing gas stream, of a gaseouscomponent selected from the group consisting of oxygen, chlorine,bromine, iodine, sulfur dioxide, sulfur trioxide, hydrogen sulfide andcarbon dioxide and a mixture thereof which comprises: providing a sampleelectrode and a reference electrode of substantially the same metal, aporous membrane, and a reservoir containing an aqueous electrolytesolution, the electrodes being separated by and each in contact with theporous membrane, the membrane extending into and being wetted by andhaVing wicking action toward the electrolyte solution and theelectrolyte solution being taken up substantiaLly throughout the extentof the membrane, said electrolyte solution containing a mobile solubleion and being not destructively reactive with the electrodes, bothelectrodes being electrically connected via potential differencemeasuring means, conducting said gas stream over the sample electrodewithout contacting the reference electrode or the electrolyte solutionin the reservoir, and measuring changes in potential difference betweenthe electrodes as a consequence of said fluctuations without impressinga potential across the electrodes.
 20. The method as in claim 19 inwhich the reference electrode is provided with a controlled environmentthat contains a concentration of the said gaseous component such thatthe reference electrode does not alternately function as cathode andanode as a result of fluctuations in concentration of the said gaseouscomponent being detected in the flowing gas stream.